1. Field of the Invention
The present invention relates to a method and apparatus for manufacturing a zinc oxide thin film at low temperatures.
2. Description of Related Art
Transparent electrode films are indispensable to electronic components of a digital camera, DVD video, plasma display panel, organic EL panel and the like, and ITO (Indium Tin Oxide) films have been used widely in the transparent electrode films. However, Indium is expensive, further has the problem of resource exhaustion, and needs to be urgently replaced with another material.
Zinc oxide is chemically stable and conventionally used harmless material, and has an advantage of low environmental load. Among principal methods of manufacturing a thin film of zinc oxide are sputtering, organic metal chemical vapor deposition (CVD), and spray thermal decomposition method. Sputtering has been used the most widely, but has problems that the film has nonuniform electric resistivity and that the nonuniformity is remarkable when the film is formed at low temperatures.
Organic metal CVD has problems of generating a large mount of waste gas and imposing heavy loads on the environment. Problems of the spray thermal decomposition method are that heat of about 550° C. is necessary to obtain the high-performance film and that the film cannot be formed at low temperatures.
Another technique to form a zinc oxide thin film other than the above-mentioned methods is Molecular Beam Epitaxy (MBE) (see “Plasma assisted MBE growth and characterization of hexagonal ZnO on GaAs (1111)” Proceeding of 1st Asia-Pacific Workshop on Widegap Semiconductors, 153-156 (2003) and “MBE growth and optical properties of ZnO on GaAs (1111) substrates” Phys. Stat. sol. (b), 241, 591-594 (2004)). However, formation of zinc oxide thin film disclosed in the these documents is to form the film at temperatures of 400° C. or more, and a problem arises that these methods cannot be applied to the case of using materials with low heat resistance such as plastic as a substrate. Another problem is that doping impurities is necessary to form the film to decrease the electric resistivity of the transparent electrode film.
Further, Japanese Patent Application Publication Nos.: 2003-89875 and JP 2001-271167 disclose the technique of forming a zinc oxide thin film using atmospheric glow discharge plasmas. Problems in the technique of both publication documents are that the plasmas are generated using inert gas and that the resistivity of the zinc oxide thin film is 3MΩ or more.
Furthermore, in the method of forming a zinc oxide thin film disclosed in these documents and the like, a substrate placed in a film-forming chamber evacuated to a vacuum is heated to 400° C. or more to accelerate the zinc oxide reaction and the film is thereby formed. Moreover, these methods do not provide sufficient conductivity, and have the need of doping impurities during formation of the film.